Electrostatically applicable coating powder and processes therefor

ABSTRACT

Hitherto, glazing defects have arisen during glazing of ceramic substrates with an electrostatically applicable coating powder. This problem is solved by the coating powder CP AB  and the coating method. The electrostatically applicable coating powder CP AB  contains 1 to 50 wt. % of a glass-forming composition A which begins softening at 400 to 750° C. and 99 to 50 wt. % of a glass-forming composition B which begins to soften above 750-1,100° C. Preferably, the coating powder consists of 2.5 to 25 wt. % A, 75 to 97.5 wt. % B and 0 to 20 wt. %, specially 2 to 10 wt. % of thermally or chemically activatable adhesive agents. The coating powder CP AB  is electrostatically applied as a single powder. Alternately, coating can be applied in two layers, wherein the CP U  powder contains at least 5 wt. % of glass-forming composition A as a lower layer and the powder CP O  contains at least 50 wt. % of glass-forming composition B as an upper layer.

BACKGROUND OF THE INVENTION

This invention relates to a process for the production of ceramic andvitreous coatings, in particular glazes and engobes, comprisingelectrostatic application of a coating powder onto a ceramic substrateand firing of the coated substrate. The invention furthermore relates toa coating powder particularly suitable for the performance of theprocess and to the use thereof.

Ceramic and vitreous coatings, such as engobes and glazes, on ceramic,in particular unfired or partially fired, substrates are predominantlyproduced using aqueous slips. After application of the slip, thesubstrate coated therewith is fired, wherein the stovable materialcontained in the slip melts or sinters together to yield a ceramic orvitreous layer. Due to the disadvantages associated with the use ofaqueous slips, such as effluent problems and elevated energyconsumption, electrostatic powder coating of ceramic products isbecoming increasingly significant.

Providing ceramic substrates, such as porcelain, earthenware andstoneware, but in particular unfired or only partially fired substrates,with an electrostatic coating still occasions various problems withregard to electrostatic application of the coating powder, inadequateadhesion of the powder to the substrate and, often, inadequate handlingresistance. In addition to these problems, there are glazing defectswhich only occur on firing.

Various approaches have been used to solving the electrostatic andadhesion problems: thus, according to DE-PS 29 41 026, glaze powder maybe adjusted to a specific surface resistance value suitable forelectrostatic application of greater than 1·10¹⁰ Ohm·m by coating with apolysiloxane. Engobes may also be applied electrostatically as powdersafter such hydrophobing treatment (EP-A 0 442 109). While, according toDE-A 42 39 541, the adhesion of an electrostatically applied glazepowder may indeed be improved by initially applying an aqueous couplinglayer containing a polymer and a glass frit onto the substrate, the useof an aqueous system is regarded as disadvantageous.

WO 94/26679 discloses an improvement to the adhesion and handlingresistance of a stovable coating powder, such as a glaze powder, whichhas been electrostatically applied to a substrate: in this case, thecoating powder contains, apart from a glaze powder, a chemically orphysically activatable coupling agent which combusts without leaving aresidue on firing, such as polyolefins and dextrins, by means of which,after activation, the particles of the layer are fixed to each other andto the substrate. Preferred coating powders contain polysiloxane-coatedglass frits mixed with to 15 wt. % of thermoplastic or 5 to 10 wt. % ofdextrin. Usable glazes may be obtained on porcelain biscuit bodies onlyunder specially optimised conditions, which, however, entail increasedcosts. If the conditions are only slightly modified, depending upon thesubstrate, sometimes considerable glaze defects and deficiencies occurbefore the required layer thickness is obtained.

WO 97/08115 discloses a remedy to the above-stated problems: theproduction process may be simplified without degrading glaze quality byusing a glaze or engobe composition having a particular grain sizedistribution, namely a d₅₀ value of 5 to 25 μm, a d₉₀ value of less than35 μm and a d₁₀ value of greater than or equal to 2 μm, in a coatingpowder additionally containing a coupling agent.

While the two above-mentioned processes do allow the production ofdefect-free glazes on fired or biscuit-fired porcelain bodies, when thesame methods are used to produce glazes and engobes on unfired bodies,such as wall and floor tiles, the stoved coating exhibits considerabledeficiencies in quality. In the case of biscuit-fired bodies too, thefrequency of glaze defects is often excessively high. The defects oftentake the form of large, “frozen-in” blisters and extensive areas withoutglaze (the occurrence of such defects is also known as “crawling”).These defects often occupy 10 to 30% of the stated area. The cause ofthis defect is suspected to involve the following interactions: theelectrostatically applied layer is much looser and moreover exhibitsmuch lower adhesion to the substrate than the layer obtained when aconventional slip is used. During firing of a green body provided with aglaze layer, gas-forming reactions occur in the green body, in theboundary layer between the body and the glaze layer and in the glazelayer, such reactions including, for example, the conversion of kaolininto metakaolin, which in particular proceeds at a temperature in therange from approx. 700 to 800° C., and the elimination of CO₂ fromcarbonates, such as dolomite, present in the body, which occurs ataround 900° C. If the layer is too thin and adheres inadequately, it islabile from when an initially present organic binder has combustedduring the firing until the temperature at which the glaze melts, suchthat it may be destroyed by slight mechanical action, such as vibrationand air currents in the kiln and by degassing processes in the body. Oneof these disruptions is manifested by lifting of an area of theelectrostatically applied layer, so resulting in the formation ofblisters and glaze-free areas once the blisters have burst. Clearly, thecapillary forces which occur when a slip is applied using conventionalmethods firmly anchor the layer to the green body, such that the stateddisruptions do not occur, whereas such anchoring is absent when thelayer is applied electrostatically.

SUMMARY OF THE INVENTION

The object of the present invention is accordingly to overcome thestated problems in the production of ceramic and vitreous coatings, inparticular glazes, wherein the coating powders are appliedelectrostatically. Once fired, the coatings may contain crystallineand/or amorphous fractions.

A process has been found for the production of a ceramic and/or vitreouscoating, in particular a glaze, on a ceramic substrate, comprisingelectrostatic application of a coating powder and firing of the coatedsubstrate, which process is characterized in that a coating powderCP_(AB) is used which contains 1 to 50 wt. % of a layer-formingcomposition A having a softening onset in the range from 400 to 750° C.and 99 to 50 wt. % of a layer-forming composition B having a softeningonset in the range from above 750 to 1100° C., or in that an underlayeris applied onto the ceramic substrate using a coating powder CP_(U)containing at least 5 wt. % of the layer-forming composition A and anupper layer is applied thereon using a coating powder CP_(O) containingat least 50 wt. % of the layer-forming composition B, wherein at leastone of the two layers is applied electrostatically and the underlayerconstitutes at least 1% of the total coating. The term “layer-forming”is taken to mean that, on firing, the composition is capable of forminga layer of a crystalline and/or amorphous, i.e. ceramic and/or vitreous,material.

The essence of the invention is the use of a layer-forming compositionA, which has a softening onset T(EB)_(A) below the softening onsetT(EB)_(B) of a layer-forming composition B, wherein composition Bconstitutes the majority of the ceramic coating to be produced. Thelayer-forming composition A preferably comprises a low-melting glasscomposition. The majority of the coating comprises compositions as areused for glazing, engobing and otherwise decorating ceramic substrates.According to the invention, the ceramic substrates to be coated inparticular comprise those in which or at the boundary layer of whichwith the applied coating degassing processes occur during firing; suchsubstrates include unfired or biscuit-fired earthenware and stoneware aswell as unfired and biscuit-fired porcelain. Unfired wall and floortiles and roofing tiles are particularly suitable substrates.Composition A is either a constituent in an effective quantity of thecoating powder CP_(AB) directly applied as a single layer or aconstituent in an effective quantity of a coating powder CP_(U) for alower layer, over which an upper layer of a coating powder CP_(O) of adifferent composition is applied.

The function of composition A is considered to be that it :effects goodadhesion of the coating to the substrate during the stoving operation.Composition A is here selected such that its softening onset ispreferably below the temperature at which degassing processes occurwithin the substrate, at the boundary layer and within the coating.Since composition A melts before the said degassing processes occur, inthe case of the single-layer structure preferred according to theinvention using the coating powder CP_(AB), both good adhesion to thesubstrate and good cohesion of the coating itself are effected duringfiring. Blistering, displacement of the entire layer, for example due tovibration in a sliding-bat kiln, or blowing off in a stationary kilnoperated by firing are thus avoided. If the optimum composition andquantity to be used are selected, the area of crawling (=total area ofdefects) may be reduced to zero. Degassing from the substrate mayproceed in part through the reverse side thereof and/or through thesintered coating.

In the alternative two-layer structure, the lower layer effects adhesionto the substrate. Even if the lower layer is much thinner than the upperlayer arranged thereon, crawling is avoided during firing. The thicknessof the lower layer is at least 1% and generally less than 50%, inparticular 2 to 30%, of the total thickness of the two layers. Applyinga low-melting glass flux as a thin lower layer has the advantage thatthe properties of the upper layers which are vital for service are muchless affected.

After firing, the total thickness of the single or two-layer ceramicand/or vitreous coating is within the conventional limits known from theprior art, for example from glazing and engobing using aqueous slips,i.e. usually in the range from 50 to 1000 μm, in particular from 100 to500 μm, and in the case of glazes particularly preferably in the rangefrom 200 to 300 μm.

While the chemical composition of the layer-forming composition A, whichin both alternative embodiments is a constituent of the layer in contactwith the ceramic substrate, does substantially determine the softeningonset and the course of melting, determined as T(EB) and T(HK)(=hemisphere point) under a heating microscope, it is, however, oflittle significance to the final result of the single or two-layercoating. Composition A may assume the form of a glass frit or of apulverulent mixture of substances, which begins to soften at T(EB) toform glass. Glass frits are preferred. When a mixture of substances isused, it conveniently assumes the form of a previously homogenised andspray-pelletised powder. It is known that low softening point glasses,which are also known as fluxes in decorative applications, arecharacterized by elevated contents of oxides from the range comprisingPbO, Bi₂O₃, ZnO, B₂O₃ and alkali metal oxides. Where an unfrittedcomposition A is used, it may consist of one or more substances from therange comprising alkali metal borates, alkali metal silicates, leadborosilicates, bismuth borosilicates and zinc borosilicates. 80 to 100wt. % of composition A, relative to the glass-forming components,preferably assume the form of a glass frit.

Composition A is conveniently used in ultra-finely ground form.Effective action is then achieved at a low usage rate if the grain sizerange is substantially finer than that of the layer-forming compositionB. The d₅₀ value of composition A is preferably in the range from 1 to 5μm and the d₉₀ value is below the d₅₀ value of composition B.

The softening onset of the composition A to be used is usually in therange from 400 to 750° C., in particular in the range from 450 to 600°C. A softening onset of around 500° C. is particularly preferred,because it is then ensured that, on firing, once an organic couplingagent which is additionally present in the coating powder and isinitially responsible for good adhesion and handling resistance of thelayer has combusted, the layer-forming composition A exerts its action.

Layer-forming composition B comprises such a composition which begins tosoften at around/above 750° C., preferably at 800 to 1050° C. and inparticular in the range from 900 to 1000° C. Composition B should notbegin to soften until composition A has melted to such an extent thatthe required adhesion to the substrate and within the coating containingcomposition B has come into effect. The chemical composition ofcomposition B corresponds to that as is conventional for coatings ofthis generic type, such as in particular glazes and engobes. The personskilled in the art is familiar with such compositions; reference is madeby way of example to Ullmann's Encyclopedia of Industrial Chemistry,5^(th) edition, 1986, pp. 31-33. Glazes which are used according to theinvention preferably comprise systems, the majority of which assumes theform of a glass frit, but a smaller proportion of which, namely up to 30wt. %, preferably up to 10 wt. %, may be present in the form of furthercomponents, such as clay minerals, such as kaolin, and/or nephelinesyenite. Such systems are in particular suitable for so-called rapidsingle firing glazes, as are used for glazing wall tiles, which arefired at approximately 1100° C. (±50° C.). Glazes for floor tiles aremore strongly formulated to a T(EB) of around/above 900° C. and a firingtemperature of around 1200° C. Such glazes conventionally contain asmaller proportion of glass frits; example compositions substantiallycontain (wt. %) 30 to 50% glass frit, 5 to 15% wollastonite (Casilicate), 5 to 15% alumina (Al₂O₃), 0 to 15% zirconium silicate, 5 to15% kaolin plus colouring pigments if required. Glazes based on theabove-stated mixtures of substances are conveniently used in the form ofsprayed pellets.

The substantial constituents of engobes are glass frits, finely dividedceramic raw materials, ground minerals, glass and porcelain flour,together with opacifiers and/or pigments. In this case too, it isconvenient to use spray-dried pellets because segregation is avoided anduniform melting behaviour is achieved.

Composition B preferably consists of 30 to 100 wt. % of one or moreglass frits.

According to a preferred embodiment, both the coating powder CP_(AB) tobe sprayed for the single layer coating and the powders CP_(O) andoptionally also CP, to be used for the two-layer coating, have a grainsize range as disclosed in WO 97/08115 of d₅₀ 5 to 25 μm, d₁₀≧2 μm andd₉₀ less than 35 μm.

The powder of both composition A and B or of the mixture containing Aand B may, where required for electrostatic reasons, have a knownhydrophobing outer coat, for example a polysiloxane outer coat.

The coating powder CP_(AB) to be used for the single layer coatingcontains 1 to 50 wt. % of composition A and 50 to 99 wt. % of acomposition B. Preferably, however, CP_(AB) contains 2.5 to 25 wt. % ofA and 97.5 to 75 wt. % of B, in each case relative to all the ceramicand/or vitreous layer-forming components. Coating powder CP_(AB)preferably contains a total of 75 to 95 wt. %, in particular 90 to 95wt. % of ceramic and/or vitreous layer-forming components; where CP_(AB)contains relatively large quantities of colouring pigments, the total ofA and B may be still lower. Compositions A and B may each contain two ormore glass frits, the T(EB) values of which are within the claimedrange. An excessively large proportion of A in CP_(AB) with anexcessively low T(EB)_(A) should be avoided because an undulatingsurface and/or pinholing of the glaze may occur. The person skilled inthe art will perform initial investigatory testing to determine theblend of composition B with composition A with regard to T(EB)_(A) andthe quantity of A relative to B which is most suitable for obtaining adefect-free glaze or engobe or decoration.

For the purposes of two-layer coating, the lower layer may have acomposition similar to that of CP_(AB) but with at most 5 wt. % ofcomposition A and up to 95 wt. % of composition B. Alternatively, thelower layer may contain exclusively a composition according to A. Inthis case, the layer thickness of the lower layer should, if at allpossible, constitute no more than 10% of the total thickness of bothlayers. The upper layer contains a layer-forming composition B as themain component (≧50%) of the coating powder CP_(O). Coating powderCP_(O) may, however, also contain a composition A, specifically in asmaller quantity than it is present in CP_(U).

It is known that electrostatically applicable coating powders,preferably glazes, may, apart from ceramic and/or vitreous layer-formingcomponents, also contain one or more chemically or physically, inparticular thermally, activated coupling agents. This is also the casefor the process according to the invention and for the coating powdersto be used for this purpose, such that these preferably contain aneffective quantity of such coupling agents. Such materials which may beused are, for example, thermoplastics and chemically curing reactionresins and moisture-activatable substances. Explicit reference is herebymade to WO 94/26679 and WO 97/08115 with regard to the selection ofmaterials, the quantity to be used and the function of the couplingagents. Suitable coupling agents which are activatable by thermaltreatment are thermoplastic homo- and copolymers having a softeningpoint in the range between 60 and 250° C., preferably between 80 and200° C. and in particular between 80 and 150° C. The thermoplasticcoupling agents preferably comprise polyolefins, such as paraffin waxand low density polyethylene (LDPE), together with acrylate andmethacrylate polymers and copolymers, polyvinyl compounds, such aspolystyrene, polyvinyl acetate, ethylene/vinyl acetate copolymers,styrene/acrylate copolymers; polyesters and copolyesters as well aspolyamides and copolyamides are also usable.

According to another embodiment, hydrophilic and hydrophobic polymersare combined together as thermally activatable coupling agents in such amanner that the glaze powder optionally still containing residualmoisture and/or the substrate to be glazed are reliably wetted.Hydrophilic polymers exhibit structural elements which are capable offorming hydrogen bridges, for example hydroxyl groups, carboxyl groupsand/or ether bridges.

It is advantageous if the diameter of the particles of the couplingagent(s) used in the coating powder is within the particle size range ofthe stovable compositions. The average particle diameter d₅₀ of thecoupling agent is particularly preferably below the d₅₀ value of thestovable compositions. A narrow grain size range and moreover aspherical grain habit of the coupling agent are particularly preferred.

Chemical activation comprises, for example, polymerisation, such ascrosslinking in the presence of a polyfunctional acrylate ormethacrylate, or a polyaddition or polycondensation reaction oftwo-component systems. Physical activation preferably comprises partialmelting of the coupling agent with subsequent cooling andsolidification. This type of activation may be effected by heating thesubstrate to be coated before, during or after electrostatic coating andmay proceed using conventional ovens or irradiation with infra-red lightsources.

It has been found that a coating powder containing one or more stovablecompositions and one or more coupling agents from the range comprisingpolyolefins, in particular polyethylene, should conveniently be appliedelectrostatically within approximately one day of the production thereofin a high intensity mixing or grinding unit, in order to avoid possibleageing-related disadvantages.

It has been found that, with regard to obtaining defect-free stovedcoatings, it is particularly advantageous to incorporate the couplingagent into the layer-forming powder by means of an intensive mixing orgrinding process. The temperature occurring during incorporation must bebelow the activation temperature of a coupling agent activatable bymelting or a chemical reaction. Particularly advantageously usableintensive mixing and grinding units contain a high speed rotatingbeater, which is operated at a rotational speed in the range from 2000to 20000 rpm, in particular at approximately 5000 to 15000 rpm. Thestated measure of incorporating one or more coupling agents into acoating powder containing one or more stovable layer-formingcompositions by means of a, high intensity mixer or high intensity millis advantageous for any coating powders which are to be appliedelectrostatically, i.e. also those which are applied onto glass or firedceramic substrates or metal. A high intensity mill from the rangecomprising beater and jet mills is preferably used for this purpose. Therotational speed of the beater mills, such as a pin mill or pin beatermill, should be as high as possible during mixing. The rotational speedwill usually be in the range from 2000 to 20000 rpm (revolutions perminute), particularly 5000 to 15000 rpm.

According to a preferred embodiment, coupling agents having a narrowgrain size range are used for the production of coating powderscontaining coupling agents, for example polyethylene wax having a grainsize range of substantially 1 to 20 μm, in particular approximately 5 toapproximately 10 μm, for 90% of the powder.

The coating powders CP_(AB) and CP_(O) and/or CP_(U) may additionallycontain pigments which are stable under the firing conditions. Pigmentcontent will not generally exceed 20 wt. %. Coloured coatings may alsobe obtained by using coloured frits.

The coating powders CP_(AB), CP_(O) and CP_(U) may additionally containauxiliaries in a quantity of generally up to 5 wt. %, but usually onlyof up to 2 wt. %, for the purposes of trouble-free processing. Examplesare fluidising auxiliaries. Fluidising auxiliaries in particularcomprise pyrogenically produced oxides, which may in turn be hydrophobedSuitable fluidising agents are, for example, silica, titanium dioxideand aluminum oxide and ZrO₂. Prior art coating powders often containsuch fluidising auxiliaries in a quantity of between 0.5 and 3 wt. %;preferred coating powders according to the invention contain 0 to 0.3wt. %, in particular 0 to 0.2 wt. %, of fluidising agent, for examplepyrogenic SiO₂ (AEROSIL® from Degussa AG), relative to stovablematerial.

Further auxiliaries optionally present in the coating powders are thosewith which the electrical properties of the powders may be modified insuch a manner that their specific electrical resistance permitstrouble-free electrostatic spraying. Examples of such auxiliaries arehydrophobing agents. The specific electrical resistance of the coatingpowders should generally be within the range from approximately 10⁹ toapproximately 10¹⁴ Ohm·m.

Application of the coating in one or two layers by electrostaticspraying proceeds in a manner known per se using a high voltage gunoperating in accordance with the corona or super-corona principle.Voltage is conventionally to 100 kV, in particular 40 to 80 kV, thecurrent 40 to 80 μA.

According to a preferred embodiment of the process, the substrate to becoated is preheated before spraying, conveniently to 100 to 250° C.,because, as disclosed in WO 97/08115, this improves the adhesion of thepowder to the substrate and so facilitates handling of the unfiredcoated substrate. Pretreating the substrate with a salt according to WO94/26679 is also possible.

The coated substrate is fired in a conventional manner in known kilns.The firing temperature is determined by the composition of the unfiredor biscuit-fired substrate and of the coating, but is conventionally inthe range above approximately 900° C. up to approximately 1450° C.,usually 1000 to 1300° C.

The present invention also provides an electrostatically applicablecoating powder CP_(AB) comprising a composition which forms a layer onceramic firing, which powder is characterized in that it contains from 1to 50 wt. % of a layer-forming composition A having a softening onset inthe range from 400 to 750° C. and 99 to 50 wt. % of a layer-formingcomposition B having a softening onset in the range from above 750 to1100° C. The coating powder CP_(AB) preferably contains 2.5 to 25 wt. %of a layer-forming composition A and 75 to 97.5 wt. % of a layer-formingcomposition B, in each case relative to layer-forming components, i.e.the sum of A and B. A particularly preferred coating powdersubstantially consists of 75 to 95 wt. % of a composition B, 3 to 10 wt.% of a composition A, 2 to 10 wt. % of a thermally or chemicallyactivatable organic polymer, 0 to 2 wt. % of fluidising agents andauxiliaries to establish the specific surface resistance, such as acarboxylic acid salt. The layer-forming compositions A and B in thecoating powder CP_(AB) preferably exhibit a d₁₀ value of 5 to 25 μm, ad₉₀ value of less than 35 μm and a d₁₀ value of greater than or equal to2 μm. The d₁₀, d₅₀ and d₉₀ values state the grain diameter at which 10%,50% and 90% of the grains pass through (determined to DIN 66141, forexample using the CILAS HR 850-B granulometer).

By applying the process according to the invention in the alternativeembodiments and using the coating powder according to the invention forsingle layer coating, it is possible to glaze, engobe or decorateunfired and biscuit-fired substrates without the hitherto obtainedextensive areas of glaze defects occurring on firing. The coating powderCP_(AB) may be produced in a simple manner by intensive mixing of theconstituents. Selection of the constituents occasions no problemsbecause they comprise raw materials or intermediates, such as inparticular glass frits, conventional in the ceramics industry, as wellas known auxiliaries. The coating powders according to the invention areparticularly suitable for the production of glazed tiles using the rapidsingle firing process, wherein unfired tiles are coatedelectrostatically and then fired (Monoporora process).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLES (E) ANDCOMPARATIVE EXAMPLES (CE)

Coating powders were produced by intensive mixing of the componentsstated in Table 1 in an intensive mixing unit (Pulverisette 14 millwithout screen attachment from the company Fritsch, Idar-Oberstein (DE))at 12000 rpm in two passes.

TABLE 1 Powder A Glaze 32366/4 (= glass frit) : 94% Polyethylene wax (=coupling agent): 5.8% Aerosil 200 (= fluidising agent): 0.2% Powder B 1Powder A + 1% flux TDF 5512a Powder B 2 Powder A + 3% flux TDF 5512aPowder B 3 Powder A + 5% flux TDF 5512a Powder B 4 Powder A + 10% fluxTDF 5512a Powder B 5 Powder A + 5% flux RD 2002 Powder B 6 Powder A + 5%flux 106053 Powder C Engobe (79/507/A) - Dry pellets: 94% Polyethylenewax: 5.8% Aerosil 200: 0.2% Powder D 1 Powder C + 1% flux TDF 5512aPowder D 2 Powder C + 3% flux TDF 5512a Powder D 3 Powder C + 5% fluxTDF 5512a Powder D 4 Powder C + 10% flux TDF 5512a Powder D 5 Powder C +5% flux RD 2002 Powder D 6 Powder C + 5% flux 106053

The individual components are:

Glaze 32366/4: Glass frit 290/498 from Cerdec AG with optimised grainsize distribution—d₁₀ 3.7 μm, d₅₀ 19.5 μm, d₉₀ 34.5 μm. The principalcomponents of the frit are SiO₂, ZnO, CaO, MgO, B₂O₃, Al₂O₃, ZrO₂ andK₂O. Softening onset T(EB) 940° C., hemisphere temperature T(HK)=1080°C. (determined under a heating microscope).

Flux TDF 5512a: Glass frit based on SiO₂, Bi₂O₃, Na₂O, K₂O, Li₂O andAl₂O₃; softening onset T(EB) 550° C., hemisphere point T(HK) 770° C.,both values determined under a heating microscope. Grain size range d₁₀0.5 μm, d₅₀ 1.4 μm, d₉₀ 6.1 μm.

Flux RS 2002: Lead-free glass frit based on SiO₂, Bi₂O₃, ZnO, B₂O₃ andNa₂O as the principal components. Softening onset T(EB) 550° C.,hemisphere point T(HK) 680° C., in each case determined under a heatingmicroscope.

Flux 106053: Glass frit based on SiO₂, ZnO, B₂O₃, Na₂O as the principalcomponents. T(EB) 510° C., T(HK) 620° C. (determined under a heatingmicroscope). Grain size range d₁₀ 0.8 μm, d₅₀ 3.0 μm, d₉₀ 11.5 μm.

Polyethylene wax: Softening temperature 135° C.; grain size range d₁₀1.8 μm, d₅₀ 10.5 μm, d₉₀ 16.9 μm.

Coating: Green bodies (15×20 cm) for wall tiles (from the companyGrohn). Spraying was performed in a coating booth. Some of the tileswere preheated to 180° C. for 30 minutes before coating, others wereused without preheating, c.f. notes to Table 2. The high voltage gun wasoperated at 70 kV and 60 μA, the quantity of dispensing air andatomising air being adjusted such that throughput was 100 g of powderper minute.

Both single and two-layer coatings were applied—see Table 2. In the caseof two-layer application, the tile provided with the underlayer washeated for 30 minutes to 180° C. once the underlayer had been applied inorder to allow the application of a sufficient quantity of glaze as theupper layer.

Firing was performed using the rapid single firing process in agas-fired chamber kiln at T_(max)=1150° C. with a total firing time of48 minutes. After firing, the area of glaze defects (crawling), relativeto the tile area, was determined on two or more tiles in each case.

TABLE 2a Glaze - single layer Powder application 21 g/tile No. PowderCrawling (%) CE 1 A 20-30   E 1 B 1 10-20 *) E 2 B 2  5-10 *) E 3 B 3 0**) E 4 B 4 0 **) E 5 B 5 0 **) E 6 B 6 0 **) *) Application in E 1 andE 2 onto cold bodies; on application onto bodies preheated to 180° C.for 30 minutes, the crawling value when powders B 1 and B 2 were usedremained virtually unchanged. **) Application in E 3 to E 6 onto bodiespreheated to 180° C. for 30 minutes. In E 3, application was performedon tiles which had not been preheated, but it proved impossible to applythe entire required quantity of coating powder.

TABLE 2b Glazes - two layers Underlayer 5 g and upper layer 16 g/tileUnderlayer Upper layer No. powder powder Crawling (%) E 7 B 1 A 20-30 E8 B 2 A  5-10 E 9 B 3 A 0  E 10 B 4 A 0 Note: Once the underlayer hadbeen applied, the tile was heated to 180° C. for 30 minutes and then theupper layer was applied electrostatically onto the heated, coated tile.

TABLE 2c Engobes - single layer 8 g/tile No. Powder Crawling (%) CE 2 C20-30 E 11 D 1 10-20 E 12 D 2 0-5 E 13 D 3 0 E 14 D 4 0 E 15 D 5 0 E 16D 6 0 Note: Powder applied on green bodies which had not been preheated.

TABLE 2d Engobes - two layers 2 g/tile in the underlayer and 6 g/tile inthe upper layer No. Powder Powder Crawling (%) E 17 D 1 C 10-20 E 18 D 2C 0-5 E 19 D 3 C 0 E 20 D 4 C 0 Note: Application without preheating thebodies

The Examples show that in both alternative embodiments of the inventioncrawling defects are distinctly decreased and finally fall to 0% in thepresence of even a very small quantity of a composition A (=flux) in thecoating powder, which is applied directly onto the substrate.

Using the production of the composition A coating powder by way ofexample, the influence of the mixing unit used was investigated: whenthe above-stated Pulverisette was used, the proportion of tilesexhibiting crawling was 30 to 40% at 8000 rpm and 2 passes, and 20 to30% at 12000 rpm and 2 passes. When a plough bar mixer (Lödige mixer)was used, the proportion of tiles exhibiting crawling was 60 to 70% at amixing time of 20 minutes and 40 to 50% at a mixing time of 50 minutes.

EXAMPLE E 21

A white zirconium frit (no. 997633) having a T_(EB) of 1020° C. and fluxfrit (TDF 5512 A) having a T_(EB) of 550° C. were each ground in a jetmill and the powders classified such that d₅₀ was 20 μm and no particleslarger than 60 μm and smaller than 0.5 μm were present.

The classified frits were mixed in 95:5 ratio by weight. 4.75 wt. % ofspherical polyethylene wax having a narrow grain size distribution(around 10 μm) and a molecular weight of 8700 and 0.2 wt. % of pyrogenicsilica (Aerosil 200® from Degussa AG) were added to the mixture andcoarsely mixed. The mixture was homogenised by two passes through a pinmill (Pulverisette) at 10000 rpm. The powder was sprayed onto unfiredclay tiles (200×150 mm) using a super-corona electrostatic spray gun—70kV, carrier air 0.5 bar, air flow rate 5 m³/h. 21 g of powder weresprayed on, then the sprayed tile was heated to 180° C. for 10 minutes.After cooling, the coating adhered firmly and resisted handling. Firingwas performed for 40 minutes in a sliding-bat kiln at a peak temperatureof 1150° C. The glaze on the tile was cohesive, defect-free and glossy.

What is claimed is:
 1. A process for forming a coating on an unfired orbiscuit-fired ceramic substrate, which comprises: coatingelectrostatically an unfired or biscuit-fired ceramic substrate with acoating powder, wherein the coating powder comprises 1 to 50% by weightof a ceramic or vitreous layer-forming composition A having a softeningonset in a range of 400 to 750° C., and 50 to 99% by weight of a ceramicor vitreous layer-forming composition B having a softening onset in arange of above 750 to 1100° C., and firing the substrate at atemperature from above 900° C. to about 1,450° C.
 2. The processaccording to claim 1, wherein the coating powder consists essentially of1 to 50% by weight of composition A, 50 to 99% by weight of compositionB, 0 to 20% by weight of one or more chemically or thermally activatablecoupling agents, 0 to 20% by weight of pigments, and 0 to 5% by weightof auxiliaries.
 3. The process according to claim 1, wherein compositionA comprises 80 to 100% by weight of one or more glass frits.
 4. Theprocess according to claim 1, wherein composition B comprises 30 to 100%by weight of one or more glass frits.
 5. The process according to claim1, wherein the coating powder comprises 2.5 to 25% by weight ofcomposition A and 75 to 97.5% by weight of composition B, based on thetotal weight of composition A and composition B.
 6. The processaccording to claim 1, wherein composition A has a softening onset in arange of 450 to 600° C., and composition B has a softening onset in arange of 800 to 1050° C.
 7. The process according to claim 1, whereinthe coating is a glaze or engobe.
 8. The process according to claim 1,wherein the ceramic substrate is unfired or biscuit-fired earthenware orstoneware.
 9. The process according to claim 1, wherein the compositionA has a softening onset in the range of 500° C. to 750° C.
 10. A processfor forming a coating on an unfired or biscuit-fired ceramic substrate,which comprises: coating an unfired or biscuit-fired ceramic substratewith an underlayer of a first coating powder comprising at least 5% byweight of a ceramic or vitreous layer-forming composition A having asoftening onset in a range of 400 to 750° C., coating the ceramicsubstrate with an upper layer of a second coating powder comprising atleast 50% by weight of a ceramic or vitreous layer-forming composition Bhaving a softening onset in a range of above 750 to 1100° C., and firingthe substrate, wherein at least one of the underlayer and the upperlayer is applied electrostatically, and wherein the thickness of theunderlayer is at least 1% by weight of the total underlayer and upperlayer coatings.
 11. The process according to claim 10, whereincomposition A comprises 80 to 100% by weight of one or more glass frits.12. The process according to claim 10, wherein composition B comprises30 to 100% by weight of one or more glass frits.
 13. The processaccording to claim 10, wherein composition A has a softening onset in arange of 450 to 600° C., and composition B has a softening onset in arange of 800 to 1050° C.
 14. The process according to claim 10, whereinthe coating is a glaze or engobe.
 15. The process according to claim 10,wherein the ceramic substrate is unfired or biscuit-fired earthenware orstoneware.
 16. The process according to claim 10, wherein both theunderlayer and the upper layer coatings are applied electrostatically,and wherein the thickness of the underlayer is 2 to 30% by weight of thetotal underlayer and upper layer coatings.
 17. An electrostaticallyapplicable coating powder, comprising 2.5 to 25% by weight of a ceramicor vitreous layer-forming composition A having a softening onset in arange of 400 to 750° C., 75 to 97.5% by weight of a ceramic or vitreouslayer-forming composition B having a softening onset in a range above750 to 1100° C., and up to 20% by weight of one or more chemically orthermally activatable coupling agents.
 18. The coating powder accordingto claim 17, wherein composition A and composition B exhibit a grainsize distribution with a d₉₀ value of less than 35 μm, a d₅₀ value in arange of 5 to 25 μm, and a d₁₀ value greater than or equal to 2 μm. 19.The coating powder according to claim 17, wherein composition Acomprises 80 to 100% by weight of one or more glass frits, andcomposition B comprises 30 to 100% by weight of one or more glass frits.20. The coating powder according to claim 17, which further includes ahydrophilic and a hydrophobic polymer as a coupling agent.
 21. Thecoating powder according to claim 17, wherein the coupling agent is athermoplastic organic polymer.
 22. The coating powder according to claim21, wherein the thermoplastic organic polymer is polyethylene.
 23. Thecoating powder according to claim 17, which further includes up to 5% byweight of one or more auxiliary.
 24. The coating powder according toclaim 23, wherein at least one auxiliary is a fluidizing auxiliary or anauxiliary for increasing specific electrical resistance.
 25. The coatingpowder according to claim 23, which contains up to 3% by weight ofpyrogenic silica as a fluidizing agent, or which contains up to 3% byweight of a salt of a carboxylic acid having 1 to 6 carbon atoms as anauxiliary for increasing specific electrical resistance, or whichcontains both.
 26. The coating powder according to claim 25, wherein thecarboxylic acid has 2 carbon atoms and is a Mg or Ca salt.
 27. A processfor producing the electrostatically applicable coating powder accordingto claim 17, which comprises mixing of the composition A, thecomposition B and the one or more coupling agents using an intensivemill selected from a beater and jet mill.
 28. The process according toclaim 27, wherein the mixing is performed using a pin or pin beater millat a rotor speed of 2,000 to 20,000 rpm.
 29. The process according toclaim 28, wherein the rotor speed is 5,000 to 15,000 rpm.